In a subsurface pneumatic steel refining process, oxygen is injected into a steel melt from below the melt surface to decarburize the melt. Subsurface injected oxygen reacts with carbon in the melt to form carbon monoxide which then bubbles up through and out of the melt, thus serving to remove carbon from the melt. The reaction of oxygen and carbon to form carbon monoxide is exothermic and this serves to give added benefit by providing heat to the melt so as to assist in achieving the desired tap temperature of the melt.
Although the reaction of oxygen and carbon to form carbon monoxide is beneficially exothermic, their reaction to form carbon dioxide is considerably more exothermic. For example, the theoretical heat generated by the reaction of one mole of carbon and one-half mole of oxygen gas to form one mole of carbon monoxide is 26.4 kilocalories, while the theoretical heat generated by the reaction of one mole of carbon and one mole of oxygen gas to form one mole of carbon dioxide is 96.05 kilocalories. These facts are well known to those skilled in the art and a number of processes have been advanced to take advantage of these chemical reaction thermodynamics in order to produce greater heat from the decarburization of a steel melt.
One such process involves injecting oxygen onto the bath surface in addition to that injected into the melt from below the melt surface. This top-injected oxygen reacts with carbon monoxide in the head space above the bath surface. This carbon monoxide, which has bubbled up through and out of the melt, then forms carbon dioxide thus generating the additional heat alluded to above in discussing the difference between the reaction of carbon and oxygen to form carbon dioxide as opposed to carbon monoxide. Also, it has been demonstrated that the combustion of carbon monoxide above the surface of a chromium containing steel melt that is decarburized by the injection of oxygen beneath the surface of the bath, supresses the the oxidation of chromium and in effect increases the rate of carbon removal without increasing the rate at which oxygen is injected into the molten bath.
Not all of the top-injected oxygen reacts with carbon monoxide in the headspace to form carbon dioxide. Some of this top-injected oxygen impacts the bath and reacts with bath constituents. Some of these bath constituents may be silicon or aluminum which may have been added to the melt to provide heat to the melt. Other bath constituents with which top-injected oxygen may react include chromium, manganese and iron. The reaction of top-injected oxygen with carbon has the beneficial aspect of assisting in the decarburization of the steel melt, thus reducing the time and hence the cost of refining any given steel melt to any given desired final carbon content.
However, this process has heretofore had the major disadvantage of introducing an uncertainty into the decarburization process. This is because the percentage of oxygen which reacts with carbon monoxide in the headspace and the percentage of oxygen which reacts with bath constituents could not be accurately predicted and controlled. When refining plain carbon steels containing less than two percent total alloying elements such as manganese and chromium, carbon is the main bath constituent that is oxidized during decarburization. Thus when refining plain carbon steels the amount of carbon removed from the steel melt could not be precisely controlled because of the uncertainty of exactly how much carbon is oxidized by the top-injected oxygen. This is not a major problem when the steel being made has a wide carbon specification. However, this process for increasing the heat generated by decarburization has severe limitations if one desires a steel with a precisely defined carbon content.
In the production of high quality low alloy or stainless steels containing greater than two percent alloying elements such as manganese and chromium, these elements are oxidized along with carbon during decarburization. Thus it is necessary to add a deoxidant to the molten bath after the desired carbon level has been obtained, in order to recover valuable metallics, such as chromium and manganese present in the slag as oxides. The deoxidant, which generally is silicon or aluminum, will combine with the metallic oxides to form aluminum oxide or silicon dioxide, leaving the valuable metallics in their elemental form such as chromium and manganese. The valuable metallics will remain in the melt while the aluminum oxide and silicon dioxide will remain in the slag. In order to effectively recover the oxidized metallics while obtaining specification silicon and/or aluminum content of the steel, it is necessary to know the quantity of top-injected oxygen that reacts with bath components.
It is therefore an object of this invention to provide an improved method of refining a steel melt by subsurface oxygen injection with secondary top-blown oxygen.
It is another object of this invention to provide an improved method of refining a steel melt by subsurface oxygen injection with secondary top-blown oxygen wherein the percentage of top-blown oxygen which reacts with bath constituents in accurately predicted and controlled.